The invention relates to a process for the forming of a hollow body and to an apparatus for carrying out the same.
A process of the generic type is known from EP 0 621 091 A1. In this case, a pre-bent tubular hollow body which is straight in the vertical plane and has a circular cross section is placed horizontally into an internal high-pressure forming moldxe2x80x94which is divided into an upper die and a lower die. In the closed position of the forming mold, the upper die forms with the lower die a mold cavity which has a polygonal cross section. Once the hollow body has been positioned, the forming mold is closed, the hollow body being squeezed together frontally during the lowering movement of the upper die in such a way that the hollow body assumes a contour roughly approximating the contour of the mold cavity and is consequently preprofiled. The hollow body is subsequently filled with a high-pressure fluid and, after an internal high-pressure has been exerted by means of the fluid, is expanded until the walls of the hollow body come to bear against the walls of the mold cavity in a form that is largely true to the contours. If, however, the hollow body has surfaces which run at an acute angle with respect to the lowering direction of the upper die and the surfaces come into frictional contact while the operation of closing the mold is still in progress, i.e. during the lowering movement of the upper die, very high friction occurs between the hollow body and the upper die during the profiling of the said surfaces until the closed position of the forming mold is reached. The friction caused as a result has the effect that the material of the hollow body is pulled apart disproportionately there, so that the thickness of the sheet metal is reduced, or a thinning of the material occurs, which may result in cracks occurring even during the preprofiling operation. In any event, the subsequent final forming by means of internal high pressure leads to bursting of the hollow body on account of the weakening of the material of the preprofiled hollow body shape. Consequently, it is not ensured that a hollow body of this type can be produced by a dependable process.
The invention is based on the object of developing a process for dependably producing a hollow body which, as a blank, has surfaces which run at an acute angle with respect to the lowering direction of the upper die and which come into frictional contact with the latter during the preprofiling. Furthermore, an apparatus with which this hollow body can be produced is to be presented.
As a result of the invention, the friction which occurs between the mold and the hollow body to be preprofiled is absorbed as rolling friction by the roller, whereby the frictional energy does not act on the hollow body but is displaced away from the latter to the mold or the upper die. There, the frictional energy is converted into rotational kinetic energy. For this reason, the stressing of the hollow body caused by the upper die during preprofiling at the critical locations, that is the surfaces running at an acute angle with respect to the lowering direction of the upper die, is significantly reduced, so that thinning of the material, which puts the process at risk, does not occur. Consequently, even hollow bodies of a geometrically complicated shape, without an identifiable symmetry, can be formed into a profiled shape in a reliable process by the process according to invention and with the apparatus according to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.